The present invention relates to a scanning electron microscope, and particularly to a scanning electron microscope that allows efficient observation of a magnified sample image (high-magnification image) and a whole sample image (low-magnification image).
In a scanning electron microscope, the object lens is conventionally used at a very short focal distance to obtain scanned images of higher resolution, as is typified, for example, by the in-lens system in which a scanned image is obtained by placing the sample between the magnetic poles of the object lens. When the object lens is used at a short focal distance and sample observation is to be performed under high magnification, there is employed a lens control method for scanning by a primary electron beam wherein deflecting coils for the scanning by the primary electron beam are arranged in two stages along the optical axis, and the deflection point of the primary electron beam is set to be in the proximity of the principal plane of the object lens. The above arrangements are provided in order to prevent distortion caused by the object lens or an increase in the beam diameter of the primary electron beam on the periphery of the scanning region. The lens control method described above is called high-magnification mode. On the other hand, when a view search under low magnification is to be performed at a stage before proceeding to high-magnification observation as described above, or the whole image of the sample is to be observed under low magnification, the following lens control method is employed to enable scanning of a wide region (low-magnification state) by a primary electron beam. That is, the exciting current of the the object lens is set to be zero or in a weak excitation state, and the scanning of the sample by a primary electron beam is performed by using a one-stage deflecting coil or two-stage deflecting coils wherein the distance between the deflection point and the surface of the sample is set to be longer than that of high-magnification mode. This lens control method is called low-magnification mode. Thus, observation in a wide magnification range from high to low magnification has been made possible by switching between two magnification modes depending on the observation magnification.
Now, in the X-ray ananlysis of the sample by means of a scanning electron microscope, X-rays occurring from within the scanning range of the primary electron beam (view) are detected to identify the constituent elements of the sample in the view by using an X-ray spectrum and collect information on how the constituent elements are distributed in the view (X-ray mapping image) and the like. An X-ray mapping image, in particular, requires not only local element mapping of the sample through high-magnification observation but also general element mapping of the sample through low-magnification observation.
In the case of X-ray observation by using a scanning electron microscope, if an X-ray detector can be placed in the proximity of the sample, the extraction angle of X-rays can be increased, and therefore highly efficient X-ray analysis is performed. However, if a reflected electron that occurs from the sample by irradiation with the primary electron beam as in the case of X-rays falls on the detection plane of the X-ray detector, it may cause an error or a failure. This problem needs to be avoided.
If the sample is placed within the magnetic field of the object lens, as in the case of the in-lens system, X-ray analysis can be performed in high-magnification mode where the object lens is used in a strongly excitated state. This is because the magnetic field of the object lens causes the trajectory of the reflected electron to go away from the detection plane of the X-ray detector. However, in low-magnification mode where the exciting current of the object lens is set to be zero or in a weak excitation state, it is not possible to generate an object lens magnetic field strong enough to cause the trajectory of the reflected electron to go away from the detection plane of the X-ray detector. Therefore X-ray analysis is difficult to perform in this case.
Methods for performing X-ray observation in low-magnification mode include a method in which the X-ray detector is moved so as to keep away from the trajectory of the reflected electron and a method in which a magnet is placed on the detection plane of the X-ray detector to cause the trajectory of the reflected electron to go away from the detection plane of the X-ray detector. However, it is difficult to adopt such methods in the in-lens system because of its structure.
According to the present invention, low-magnification mode capable of X-ray observation is set in addition to the conventional low-magnification mode suitable for sample image (secondary electron image) observation, and X-ray analysis or particularly X-ray mapping images with a wide view can be obtained by switching between these modes.
According to the present invention, there are provided a first low-magnification mode wherein the current of the object lens is set to be zero or in a weak excitation state, and a second low-magnification mode wherein the current of the object lens is set to be a value that changes in proportion to the square root of the accelerating voltage to be used. A scanning electron microscope according to the present invention is thus provided with a configuration that makes it possible to switch to the first low-magnification mode when normal sample image (secondary electron image) observation is performed and switch to the second low-magnification mode when X-ray analysis is performed.
Specifically, a scanning electron microscope according to embodiments of the present invention comprises an electron source, a first focusing lens for focusing a primary electron beam emitted from the electron source, an object lens diaphragm for removing an unnecessary region of the primary electron beam focused by the first focusing lens, a second focusing lens for focusing the primary electron beam that has passed through the object lens diaphragm, an object lens for focusing the primary electron beam focused by the second focusing lens on a sample, a deflecting means for the scanning of the sample by the primary electron beam, a secondary electron detector for detecting a secondary electron emitted from the sample due to electron beam irradiation, and an X-ray detector for detecting an X-ray emitted from the sample. The scanning electron microscope has functions of focusing the primary electron beam on the sample by using the object lens when the magnification of an image to be scanned is higher than a preset value (high-magnification mode), and focusing the primary electron beam on the sample by using the second focusing lens when the magnification of an image to be scanned is lower than a preset value (low-magnification mode. The scanning electron microscope also has a configuration that, in low-magnification mode, makes it possible to switch to a first low-magnification mode in which the exciting current of the object lens is set to be a constant value independently of the accelerating voltage of the primary electron beam, and switch to a second low-magnification mode in which the exciting current of the object lens is changed as a function of the accelerating voltage of the primary electron beam.
The object lens diaphragm limits the focusing angle (aperture) of the primary electron beam on the sample. In addition, the control of the probe current is performed through the control of the focusing conditions of the first focusing lens. The first focusing lens or the second focusing lens may be formed by a lens in one stage or lenses in a plurality of stages.
The sample is placed in the magnetic field of the object lens. For this kind of object lens, there is known a type of object lens called an in-lens or a type of object lens called a snorkel lens.
The first low-magnification mode sets the exciting current of the object lens to be zero or in a weak excitation state. In other words, the first low-magnification mode sets the exciting current of the object lens to be the minimum exciting current of the object lens that does not lower the efficiency of secondary electron detection, or the exciting current of the object lens that provides the maximum view for the observation magnification. The exciting current of the object lens in the second low-magnification mode is set to be a value in proportion to the square root of the accelerating voltage of the primary electron beam.
The first low-magnification mode and the second low-magnification mode can be configured in such a way that switching between the first low-magnification mode and the second low-magnification mode is performed automatically according to the set value of magnification in low-magnification observation. In the first low-magnification mode, the exciting current of the object lens is lower than that of the second low-magnification mode, and the brightness region is wider than that of the second low-magnification mode (Observation under lower magnification is possible). Therefore, the range of observation magnifications is widened by setting a threshold value of observation magnification in the scanning electron microscope in advance so that it switches to the first low-magnification mode if a desired observation magnification is lower than the threshold value, and it switches to the second low-magnification mode if a desired observation magnification is higher than the threshold value.
For example, because it is easy to correct the angle of image rotation and X-ray observation is possible, the scanning electron microscope can be used in such a manner that it selects the second low-magnification mode when normal low-magnification observation is performed, and it automatically switches to the first low-magnification mode if observation is to be performed under lower magnification.
In addition, it is desirable to have the scanning electron microscope configured in such a manner that it has storage means that each stores setting values of brightness and contrast of the sample image independently for the high-magnification mode, the first low-magnification mode, and the second low-magnification mode, and according to switching to each of the magnification modes, the setting values of brightness and contrast for each of the magnification modes are automatically set to be the values stored in the storage means.
It is desirable that the deflecting means have a function of controlling the scanning direction of the primary electron beam, and the scanning direction of the primary electron beam be controlled according to switching between high-magnification mode, the first low-magnification mode, and the second low-magnification mode. It is desirable that the control of the scanning direction of the primary electron beam by the deflecting means be performed in such a way that the scanning direction of the primary electron beam on the sample substantially corresponds to the X direction of the sample stage.
When sample observation is to be performed by using the scanning electron microscope disclosed in the embodiments of the present invention, the setting value of the exciting current of the object lens in the first low-magnification mode is usually set to be a value for weak excitation, and the value is switched to zero when the scanned image of the sample is to be recorded. The scanned image of the sample is recorded by taking an image shown on a display, storing or outputting the scanned image of the sample as a file.
In addition, when sample observation is to be performed, observation of an X-ray mapping image in low-magnification mode is performed in the second low-magnification mode.
A scanning electron microscope disclosed in embodiments of the present invention comprises an electron source, a first focusing lens for focusing a primary electron beam emitted from the electron source, an object lens diaphragm for removing an unnecessary region of the primary electron beam focused by the first focusing lens, a second focusing lens for focusing the primary electron beam that has passed through the object lens diaphragm, an object lens for generating a magnetic field at the position of a sample and for focusing the primary electron beam focused by the second focusing lens on the sample, an electron beam deflecting means for the scanning of the sample by the primary electron beam, and an X-ray detector for detecting an X-ray emitted from the sample due to electron beam irradiation, whereby an X-ray mapping image of the sample is obtained. The primary electron beam is focused on the sample by the object lens to perform scanning when the magnification of an image to be scanned is higher than a preset value, and the primary electron beam is focused on the sample by the second focusing lens to perform scanning when the magnification of an image to be scanned is lower than a preset value. The scanning electron microscope has a configuration that sets the exciting current of the object lens to be in a weak excitation state to prevent the incidence of a reflected electron from the sample on the X-ray detector.
Here, if the magnification of an image to be scanned is lower than the preset value, the exciting current of the object lens is changed in proportion to the square root of the accelerating voltage of the primary electron beam.